Exposure of men to certain environmental, and medical toxicants results in prolonged azoospermia. Occasionally spermatogenesis recovers, indicating that stem (type A) spermatogonia survived but their differentiation was limited. Testes of irradiated or dibromocholoropropane (DBCP) treated rats contain A spermatogonia that remain for over one year but fail to differentiate, and may be a model for the human situation. In the rat, transient suppression of testosterone results in progression of spermatogenesis and restoration of fertility. The hypothesis is proposed that, in toxicant-treated rats, greater restoration of fertility can be achieved by manipulating various steroid hormones and FSH, which are regulating spermatogonial development by acting on Sertoli cells and possibly another steroid-receptor positive somatic cell of the testis to modulate the expression of specific genes. Initially, irradiation will be used in all the Aims, but the generality of major findings to other toxicants will be checked using DBCP-induced gonadal damage. To determine whether enhanced and prolonged recovery can be achieved, rats will be treated both before and after toxicant exposure with different combinations of a GnRH antagonist, testosterone, estrogens, or progestins. The effects of toxicant dose, timing of hormones, and specific hormone treatment on the duration of recovery will further elucidate inhibitory mechanisms and suggest procedures for restoration of spermatogenesis. To evaluate the roles of different steroid-responsive cells (Sertoli, peritubular, Leydig, vascular), three approaches will be used. These are in vitro culture of tissue fragments, isolated tubules, and tubule components; in vivo elimination of Leydig cells; and determining whether the edema (likely from vascular damage) in toxicant-treated testes correlates with the inhibition of spermatogonial development. Initial steps will be taken to identify genes involved. RNA from purified populations of the target cell from irradiated-only and hormone-treated irradiated rats will be subjected to subtractive hybridization to obtain libraries of differentially expressed clones. This study should define hormones or other factors that might be used as intervention to enhance recovery of fertility in men following certain types of toxicant exposure, cancer of immunosuppressive therapy, aging, and idiopathic infertility that result in blocks in germ cell development.